Context: The forward lunge is widely used among athletes for training and rehabilitation purposes. The forward lunge movement has also been suggested as a model to study functional adaptation to ACL rupture. Previous investigations indicate that the absence of the ACL influences the movement pattern of many patients during a forward lunge, while direct measurements of ACL strain show that except for cases close to full extension, quadriceps activity does not seem to influence the ACL strain. The question is whether there are other external forces present in the lunge movement that may cause an anterior force on the tibia, requiring the ACL to be intact to stabilize the knee?
Objective: To establish a musculoskeletal model of the forward lunge to computationally investigate the force equilibrium in the knee during forward lunge and answer the following questions: 1) Which structures in the knee are loaded during forward lunge? 2) Does the mechanical equilibrium cause ACL loads?
Design: Computational modeling.
Setting: The biomechanical forward lunge model was based on experimental motion capture data.
Patients or Other Participants: One healthy female subject (height 5 169 cm, weight 5 59.6 kg, age 5 20 years).
Interventions: Three-dimensional coordinates of skin-mounted markers were obtained via five video cameras. The subject performed a forward lunge on a force plate, targeting a knee flexion angle of 906. A model of the lunge movement was developed using the AnyBody Modeling System. The model and the pelvis. The hips were modeled as spherical joints, the knees as hinge joints, and the ankles as universal joints. Each according to a minimum fatigue criterion.
Main Outcome Measures: Muscle and joint reaction forces that pulled the tibia in anterior or posterior direction. The forces were normalized in terms of the total anterior and posterior force.
Results: No stabilization by the ACL was needed during the forward lunge. Quadriceps pulled the tibia anteriorly by less than 25% (420 N) in the beginning and the end of the movement, while it created a posterior drag of 3% (298 N) on the tibia at the time of peak knee flexion. At peak knee flexion, the knee reaction was the only force that pulled the tibia anteriorly (2880 N). This was primarily counterbalanced by the musculus gluteus maximus (21940 N).
Conclusions: The loading of the knee joint during lunging never required any stabilization by the ACL. The forward lunge explored the muscle and reaction forces, which can be used for further examination of ACL injury mechanisms and prevention strategies by applying parameter and optimization studies to the model.